1. Field of the Invention
This invention pertains generally to biomedical implantable functional or neural stimulation (FES/FNS), and more particularly to active charge balancing circuits for stimulator circuits.
2. Description of Related Art
Functional electrical stimulation is used in many biomedical implants to restore lost function in individuals by delivering current charge into biological tissues to evoke action potentials. Application examples of this technology include cochlear implants, retinal prosthesis, cortical stimulators, spinal cord implants and deep brain stimulators. These implantable devices typically deliver bi-phasic current to depolarize the neural membrane, aiming to maintain a zero charge residual at the stimulation site.
However, it should be realized that a perfect balance of current pulse amplitude is challenging to achieve due to the intrinsic mismatch of current sink and source drivers (e.g., between 1% and 5%), and interestingly, even with perfectly matched cathodic and anodic current pulses, a condition of zero residual charge is still not achievable due to the inter-pulse delay typically adopted in modern biphasic stimulation.
Numerous methods have been proposed to achieve a safe charge-balanced electrical stimulation. A common passive solution is to insert a DC-blocking capacitor in series with the stimulation electrode. This ensures that only a small DC current (<1 nA) can flow through the electrode. However, to ensure that the voltage drop across the electrode does not significantly increase the compliance voltage required to power the drivers, a physically large capacitor is usually unavoidable. Additionally, one capacitor is required per electrode. It will be appreciated that applying DC-blocking capacitors in a high-density neuron implant, such as a retinal prosthesis, would lead to an overly large physical size that would be impractical for clinical applications.
An alternative passive approach toward achieving charge-balanced stimulation is to short the stimulation electrode to the reference electrode after each stimulation period. This approach can be applied with low-frequency stimulation patterns, however, at high frequency, the time available to discharge the residual charge may be insufficient due to the large capacitance contributed by the reference electrode and usually results in a net DC charge.
Active charge balancing schemes have also been proposed. The pulse insertion technique involves inserting predefined, current pulses at the end of each stimulation pattern. So far, the efficacy of balancing short pulses has not yet been investigated utilizing clinical animal and human tests. There is a possibility that the inserted short pulses might result in unwanted neural responses.
Active offset-cancellation schemes have been similarly proposed. Active offset cancellation is implemented by continuously applying a small DC current to match the anodic and cathodic stimulus, however, a certain amount of settling time is still required for the control loop and inevitably this sets a limitation on stimulation frequency. Furthermore, one principle drawback of this approach is obtaining sufficient resolution of available calibration current, as a more precise calibration current necessitates a more complex hardware implementation.
Accordingly, a need exists for an improved apparatus and method for performing charge balanced functional stimulation. The present invention fulfills that need and overcomes shortcomings of previous functional stimulation implementations.